Please follow the below steps:
1. Navigate to Purchasing Super User Resp > Purchase Orders > Purchase Order Summary
2. Query the PO you want to open
3. Click on Tools > Control and choose "Open for Receiving"
This will open all the lines, if you wanty to open a specific line the press on lines button and click on Tools > Control
You can also open by shipment as well by pressing on shipment button and clicking Tools > Control
Hope this helps,
Please mark replies "Correct" or "Helpful" as applicable to facilitate solutions to other community participants
Or another approach is make an RTV (Return to Vendor) transaction so that the POs open up again to be received.